Optical depth will depend on the moisture density and vertical depth (thickness) of a cloud. This will influence how a cloud looks on VIS (visible), IR (infrared) and WV (water vapor) imagery. Optical depth (optical thickness) is how transparent a cloud is or how much the cloud modifies light passing through it. A thick cloud is optically thick especially if it has a high density of moisture. A thin cloud or a very cold cloud (low moisture density) is optically thin. An optically thick cloud can not be seen through as easily. An optically thin cloud can be seen through easier.

Think of optical depth as how light interacts with a cloud. Take for example the difference between thick fog and thin fog. The optical depth of the thick fog is labeled as being greater since it can not be seen through as easy. In the case of a thicker clouds, light has more cloud it must pass through to get to the other side of the cloud. Thus, optical depth is not just a distance but also how light and radiation penetrates a cloud.

Thick clouds: There is more reflection on visible satellite imagery, it is brighter white on visible satellite imagery, less sunlight penetrates the cloud (more is reflected or absorbed by cloud), and it is difficult for satellite to sense land or other clouds underneath optically thick clouds. Infrared satellite imagery can get a better assessment of the cloud top temperature with optically thick clouds since there is less radiation passing through the cloud from the surface to contaminate the cloud top temperature reading the satellite takes.

Thin clouds: There is less reflection on visible satellite imagery, more light penetrates through the cloud, some radiation can pass from the surface and through the cloud on infrared satellite imagery, and the satellite can sense land and other clouds under optically thin clouds (especially cold thin clouds since they do not have as much moisture).